As an indexable cutting tool, high-feed cutting of feeding an end mill body serving as a tool body at a high feed rate to cut a workpiece is known. As a cutting insert attached to the indexable end mill for such high-feed cutting, Patent Citations 1 and 2 suggest: a negative type cutting insert having a substantially equilateral triangular plate shape in which front and back surfaces having an equilateral triangular shape selectively serve as rake faces; corner edges having a circular arc shape are formed on corners of the front and back surfaces; and the main cutting edges are formed on side ridges of the front and back surfaces that stretch to the corner edges.
This type of cutting insert is attached to a tip of an end mill body to be rotated around an axis in a manner such that one of the front and back surfaces is directed to an end mill rotational direction as a rake face, one of the main cutting edges of the rake face is directed to a tip side of the end mill body, and a relatively small cutting edge angle, a negative axial rake angle, and a radial rake angle are given to the main cutting edge. The cutting tool is usually fed at high feed in a direction orthogonal to the axis and is used for cutting. Since the cutting edge angle of the main cutting edge is relatively small, the thickness of a chip created decreases even though the feeding is high as above, and efficient cutting can be performed.
Incidentally, in Patent Citations 1 and 2, Patent Citation 1 describes that the main cutting edge is formed in a polygonal line shape including straight lines intersecting each other at an obtuse angle, and the cutting insert described in Patent Citation 2 is formed so that the main cutting edge is convexly curved. However, if high-feed cutting is performed by such cutting inserts, the cutting edge angle becomes larger toward the outer peripheral side of the end mill body (as moving outward in the radius direction of the end mill body). Therefore, the thickness of chips created increases toward the outer peripheral side.
Since the thickness of the chip becomes maximum on a cutting boundary of an outermost periphery of the main cutting edge, there is a concern that the main cutting edge of the cutting insert may cause abnormal damage, such as chipping and breakage, on the cutting boundary. For this reason, the feed rate per one edge is suppressed to prevent the thickness of a chip from becoming excessively thick, and it is difficult to increase the feed rate to perform more efficient high-feed cutting.